Personally I'd wait until more is known on the GT200 refresh if you are gravitating towards a single gpu solution. In fairness though, given the speed difference between the 4870 and 280, in the games that do scale you'll be ahead of the 280 and in those that don't you won't be far behind and given this is for only slightly more money ( I'm guessing the rumored $499 price is for the 1GB version of the X2 and that the 2GB model will be $50-60 more), thats not a bad deal.
I found this quite opposite (I think I'm optimist in general). 4870x2 will be faster than GTX280 in NEW games as those will get CF profiles pretty fast (GRID, AoC, Crysis, CoD4, Bioshock etc.). For old games though which won't never see CF profile, it doesn't really matter because those doesn't need so much grunt for hardware than newer games. And even then it'll be only a little slower than GTX280.

So it's most likely win/win situation ...if the price is "right". For me, of course.